You could try "open collector RS232". Have a diode with the cathode
connected to TXD on each PIC. The anodes are all tied together. A single
pull-up resistor goes to the common anodes. Run this through an RS232
transmitter (like the MC1488 or equivalent). Each PIC can pull the common
line low to send a space. Each pic idles high when not transmitting. Have
an RS232 receiver (MC1489 or equivalent) receive data off the host PC.
This drives all the PIC RXD pins. The host sends commands to the PICs to
have them transmit. They only respond when requested, avoiding contention.

You can get fancier by packetizing the data, including to and from
addresses. Then both transmit and receive data could run on the same open
collector RS232 line (have the host also drive this line by having a diode
to the open collector data line after the RS232 receiver).

Harold

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